1. Field of the Invention
The present invention relates to a semiconductor rectifier and a fabrication method thereof.
2. Description of the Related Art
Various methods have been suggested for fabricating a semiconductor rectifier having excellent rectification properties, i.e., a low voltage requirement for current in the forward direction and a minimal reverse current.
FIG. 1 is a sectional view of a conventional semiconductor rectifier in which an ohmic contact metal layer 14 and a Schottky contact metal layer 12 constitute an upper electrode of a rectifier formed in and on a substrate 10. An ohmic contact 16 forms the lower electrode of the rectifier. When a reverse bias or no bias is applied to the semiconductor rectifier, depletion regions shown by dashed lines in FIG. 1 block current flow between the upper and lower electrodes. FIG. 2 is a sectional view showing the shape of the depletion regions and a forward current flow when a forward bias is applied to the conventional semiconductor rectifier of FIG. 1. FIGS. 1 and 2 are based on U.S. Pat. No. 5,306,943 (Ariyhoshi et al.) which describes a method for improving rectification properties using a Schottky diode.
When a reverse bias or no bias is applied to the rectifier, the depletion regions extend into substrate 10 from the interface between Schottky contact metal layer 12 and substrate 10 as shown in FIG. 1. The depletion regions also extend laterally from below the Schottky contact metal layer 12 to below ohmic contact metal layer 14. As a result, the depletion regions block current from ohmic contact metal layer 14. Thus, a reverse current cannot flow between the electrodes on the surfaces of substrate 10.
When a forward bias is applied to the rectifier, as shown in FIG. 2, the depletion regions shrink. Accordingly, the depletion regions below Schottky contact metal layer 12 are shallower than in FIG. 1, and the depletion region extending below ohmic contact metal layer 14 is absent, resulting in a current path from ohmic contact metal layer 14. Thus, when a forward bias is applied, forward current (indicated by the arrows in FIG. 2) flows between ohmic contact metal layer 14 and ohmic electrode 16.
Also, the depletion region formed when a reverse bias or no bias is applied has a lateral width which is not greater than the longitudinal depth. That is, the depletion region extends at least as far in the longitudinal direction (into substrate 10) as in the lateral direction. Thus, to completely block the flow of the reverse current when a reverse bias or no bias is applied, width L1 of ohmic contact metal layer 14 must be small enough that the depletion regions extending in the lateral direction can completely block the current path from ohmic metal layer 14.
However, if ohmic contact metal layer 14 is too small, high forward voltage is required to remove the depletion region formed below the ohmic contact metal layer 14 and form a forward current path. Thus, a small ohmic contact metal layer 14 hinders forward current flow.
In view of the competing requirements, the size of ohmic contact metal layer 14 should be optimized to be small enough to prevent a reverse current and still large enough to allow current flow at a low forward voltage. In general, formation of ohmic contact metal layer 14 requires a fine photolithography technology because ohmic contact metal layer 14 is optimally about 0.5 xcexcm.
The conventional rectifier described with reference to FIGS. 1 and 2 can use a Schottky diode to improve rectification properties. However, the Schottky contact metal layer and the ohmic contact metal layer require different materials that are parts of the same layer (upper electrode). This complicates the manufacturing process. Also, because fine photolithography is needed to create an ohmic contact of the optimal size, processing margins are small.
According to an aspect of the present invention, a semiconductor rectifier includes a substrate of a first conductivity type and a current path layer of a first conductivity type formed near the surface of the substrate. The current path layer has an impurity concentration higher than that of the substrate. A current block layer of a second conductivity type laterally encloses the current path layer and is deeper than the current path layer. The current block layer has an impurity concentration higher than that of the current path layer and a gap in a portion of the substrate below the current path layer. The current path layer is small enough for the portion below the current path layer to be completely blocked by the depletion region formed around the current block layer when a reverse bias or no is applied to the rectifier. For example, the size of the current path layer is equal to or less than about two times a lateral width of the depletion region. The current path layer has a plan view with a shape such as a circle, a hexagon or a bar.
First and second metal layers contact the upper and lower surfaces of the substrate, respectively, and form terminals of the rectifier. Both of the first and second metal layers may contain an ohmic contact material. Alternatively, the first metal layer contains a Schottky contact metal material, and the second metal layer contains an ohmic contact metal material.
According to an aspect of the present invention, a method for fabricating a semiconductor rectifier, includes forming a current path layer of a first conductivity type and a current block layer of a second conductivity type in a semiconductor substrate of the first conductivity type. The current path layer has an impurity concentration higher than that of the substrate and is near the surface of the substrate. The second conductivity type laterally encloses the current path layer and has a depth deeper than the depth of the current path layer. The impurity concentration of the current block layer is higher than the impurity concentration of the current path layer. For electrode formation, the method includes forming a first metal layer and a second metal layer on the substrate""s upper and lower surfaces, respectively.
In one embodiment, forming the current path layer and the current block layer includes: (b1) implanting impurities of the first conductivity type with a high concentration near the surface of the substrate to form a first impurity layer; (b2) thermally processing the resulting structure to diffuse the impurities within the first impurity layer, resulting in formation of the current path layer; (b3) implanting impurities of the second conductivity type with a concentration higher than the current path layer, near the surface of the substrate to form a second impurity region; and (b4) thermally processing the resulting structure to diffuse the impurities within the second impurity layer, resulting in the current block layer enclosing the current path layer. Alternatively, forming the current path layer and the current block layer includes: (b1) implanting the impurities of the first conductivity type with a concentration higher than that of the substrate, near the surface of the substrate, to form a first impurity layer; (b2) implanting the impurities of the second conductivity type with a concentration higher than that of the first impurity layer, near the surface of the substrate to form a second impurity layer; and (b3) thermally processing the resulting structure to form the current path layer and the current block layer which is deeper than the current path layer and encloses the current path layer. With either method, the second impurity layer typically overlaps the first impurity layer, and the size of the resulting current path layer depends on lithography which defines the second impurity layer with portion not overlapping the first impurity layer and diffusion which narrows the non-overlapped portion down to the final size of the current path layer. Accordingly, a small current path layer can be achieved with good processing margins that are not dependent on fine photolithography procedures.
Preferably, the size of the current path layer is such that the portion below the current path layer is completely blocked by the depletion region formed around the current block layer when a reverse bias or no bias is applied to the rectifier. For example, the width of the current path layer exposed by the current block layer may be equal to or less than two times a lateral width of the depletion region formed enclosing the current block layer when a reverse bias or no bias is applied to the rectifier.
According to another embodiment of the present invention, a semiconductor rectifier includes: a high-concentration substrate of a first conductivity type; a low-concentration epitaxial layer of the first conductivity type, formed on the substrate; a current path layer of the first conductivity type, formed near the surface of the epitaxial layer and having an impurity concentration higher than the epitaxial layer; a current block layer of a second conductivity type, laterally enclosing the current path layer, the current block layer having an impurity concentration higher than the current path layer and a depth deeper than the current path layer; a buried layer of a second conductivity type, formed below the current block layer and near the interface between the substrate and the epitaxial layer, the buried layer having an impurity concentration higher than that of the current path layer; and first and second metal layers formed contacting the surface of the epitaxial layer and the lower surface of the substrate, respectively.
The size of the current path layer is typically such that the portion below the current path layer is completely blocked by the depletion region formed to enclose the current block layer when a reverse bias or no bias is applied to the rectifier. For example, the width of the current path layer is equal to or less than two times a lateral width of the depletion region. Also, the buried layer surrounds an area below the current path layer, and the depletion region around the buried layer completely blocks current flow through that area when a reverse bias or no bias is applied to the rectifier. For example, the surrounded area has a width that is equal to or less than two times a lateral width of the depletion region.
According to another aspect of the present invention, a method for fabricating a semiconductor rectifier includes: implanting impurities of a second conductivity type near the surface of a high-concentration substrate of a first conductivity type, to form a high-concentration buried layer; forming an epitaxial layer of the first conductivity type on the substrate, with a concentration lower than that of the substrate; forming a current path layer of the first conductivity type with a concentration higher than that of the epitaxial layer, near the surface of the epitaxial layer; forming a current block layer of the second conductivity type with an impurity concentration higher than that of the current path layer, which laterally encloses the current path layer and has a depth deeper than the current path layer; and forming a first metal layer and a second metal layer on the upper surface of the epitaxial layer and the lower surface of the substrate, respectively.
According to the present invention, a semiconductor rectifier has a low forward voltage drop, a low reverse current, and a simple manufacturing process that provides a large processing margin.